This application is a companion to the Bonino, et al., U.S. application Ser. No. 09/111,574 filed Jul. 8, 1998, and assigned to the same assignee as the present invention.
The present invention concerns an overload protector for a force-measuring device, in particular a balance, comprising a load receiver that has two parts, the first of which serves to introduce the force to be measured into the force-measuring device. The second part, which serves to transfer the force to be measured to a measuring transducer, is connected to the first part in an arrangement resembling a parallelogram by two parallel guide members whose lengthwise direction runs transverse to the direction of the force introduction. The parallel guide members are rigid relative to their longitudinal direction and elastically flexible in the transverse direction. Formed on the first part and the second part, respectively, the overload protector has a first and a second engagement area by which the first part and the second part can be brought into an engagement that blocks their displacement relative to each other in the direction of the force introduction. The overload protector further comprises a pre-tensioned elastic element urging the two parts into spring-loaded contact opposing the force to be measured that is introduced into the first part.
The purpose of overload protectors of this kind, particularly in sensitive balances, is to protect the measuring transducer and the parts that serve to transfer the force to be measured to the measuring transducer against being overloaded by forces that significantly exceed those that occur at the specified nominal capacity load of the balance. Harmful force effects of this kind occur, for example, when through incorrect operation of the balance the load to be weighed is put on the weighing pan in an unduly abrupt manner. In this case, the instantaneous force acting on the load receiver will exceed the nominal capacity force. Even if a stationary stop is provided to limit the amount of travel of the load receiver in the direction of force introduction, the load receiver is nevertheless subjected to an excessive amount of acceleration up to the point where the motion comes to rest at the stationary stop. This gives rise to inertial forces of a corresponding magnitude in the measuring transducer as well as in the parts that serve to transmit the force to be measured; and/or it will cause transient peaks of tensile and compressive stress in the pivots and couplings, which the balance is not equipped to withstand, being designed for a given capacity load.
In a known overload protector provided in a balance as described above at the outset (DE 28 30 345 C3), the two guide members of the overload protector are formed in a triangular or trapezoidal shape out of flat material stock and are attached with clamping bolts to the second part at the longer base side of the triangle or trapeze and to the first part at the opposing corner or shorter side of the triangle or trapeze, respectively. For the purpose of attaching the guide members, the second part on the side facing away from the first part is equipped with a console located at a distance from the second part and extending in the transverse direction relative to the two guide members. The main portion of the second part that supports the console extends through the material-free interior area of the triangle or trapeze that is delimited by the sheet material portions that form the guide members running along the sides of the triangle or trapeze that extend between the two parts. This geometry of the overload protector requires a relatively large amount of space. In addition, the cost of assembling the first part and the second part and the guide members with the clamping bolts is relatively high.
On the other hand, a particularly space-saving arrangement is known for a monolithic design (DE 41 19 734 A1) of the parts that serve to introduce and transfer the force to be measured to the measuring transducer. However, this does not include an overload protector for the load receiver.
The object of the present invention is to create an overload protector of the kind described above at the outset, combining an exceptionally space-saving design with a low assembly cost.
According to the invention, the solution for this problem is to design the two parts and the two guide members as integrally connected material portions of a monolithic material block in which the guide members are separated from each other by a material-free space that traverses the material block.
Because the two parts of the load receiver are connected through the guide members into a single integral part, the assembly labor for the inventive overload protector is reduced to the operation of inserting the pre-tensioned elastic element that urges the two parts into spring-loaded contact against each other. Furthermore, the volume taken up inside the material block by the material-free space separating the two parts and the two guide members from each other can be kept small and, consequently, the amount of space required for the overload protector will also be small. Economical manufacturing processes are available for forming the material-free space including, e.g., milling, drilling or spark erosion as well as a combination of these processes. Primarily the last-mentioned, spark erosion, proves to be particularly well suited.
As long as the pre-tensioned elastic element urges the two parts into spring-loaded contact against each other, the first part and the second part are rigidly coupled to each other, and the force introduced into the first part is transmitted by the second part and directed to the measuring transducer. On the other hand, if the force introduced into the first part exceeds the contact force corresponding to the pre-tension of the elastic element, the first part will be displaced and set into motion relative to the second part up to the point where the first part comes to rest against a stationary stop. However, the portion of the inertial forces that exceeds the amount of the pre-tension is not transmitted to the second part. Thus, the harmful inertial forces are prevented from reaching the measuring transducer and those other parts of the force-measuring device that serve to transmit the force. In this, the amount of pre-tension of the elastic element is selected in the appropriate magnitude to maintain the spring-loaded contact of the two parts up to the nominal capacity load of the balance.
In a further development of the invention, a practical solution is offered wherein the first engagement area is formed by a first shoulder in the material portion making up the first part and the second engagement area is formed by a second shoulder in the material portion making up the second part. Each of the shoulders projects towards the respective opposite material portion. The first shoulder has a free surface facing towards the first guide member, i.e., against the direction of the force to be measured, and the second shoulder has a free surface facing towards the second guide member, i.e., in the same direction as the force to be measured. The material portions forming the first part and the second part are pressed against each other at the free surfaces by the pre-tensioned elastic element. Given that on the one hand, the free surface of the shoulder of the second part faces in the direction of the force introduction, i.e., the surface vector of this free surface has the same direction as the force introduction, and on the other hand, the free surface of the shoulder of the first part faces in the direction against the force introduction, i.e., the surface vector of this free surface has the opposite direction of the force introduction, the force acting on the first part will have the tendency to separate the free surface of the shoulder of the first part from the free surface of the shoulder of the second part, whereby the first part is being displaced relative to the second part in the direction of the force introduction. However, this displacement occurs only when the applied force exceeds the pre-tension of the elastic element that urges the two free surfaces into compressive engagement in opposition to the applied force.
As a preferred feature, the shoulder of at least one of the two parts is designed to allow the shoulder to be displaced in relation to the part in the direction transverse to the force introduction.
As is known, the force to be measured that is acting on the load receiver also produces a torque causing an albeit ever so slight distortion of the load receiver. As an example, this problem, known as lengthwise-eccentric loading, has an increasingly stronger effect the more a weight is placed off-center on the weighing pan that is supported by the load receiver. The distortion caused by lengthwise-eccentric loading can cause a slight slippage of the mutually engaged shoulders of the first part and the second part relative to each other. This leads to a hysteresis error. With the shoulder being configured to be displaceable in the direction transverse to the force introduction on at least one of the two parts of the load receiver, the respective shoulder can follow the distortion caused by the eccentric loading, whereby the slippage at the engagement of the shoulders together with the associated hysteresis will be avoided. To accomplish this purpose, it is sufficient if one of the two shoulders of the first or the second part is configured to be displaceable in the transverse direction. However, it is also conceivable that both shoulders be configured to be displaceable in the transverse direction relative to the force introduction.
In an advantageous further development of the invention, the displaceable shoulder is formed in a material portion that is delimited by material-free spaces and connected to the part comprising the displaceable shoulder by a thin portion designed to bend elastically in the direction transverse to the force introduction. This configuration allows the displaceable shoulder to be formed on the respective part in a space-saving manner and without adding to the cost of assembly.
In addition, it has proven to be practical if the pre-tensioned elastic element is a pre-tensioned compression spring. The required amount of pre-tension of the compression spring is achieved by compressive deformation, thereby reducing the amount of space occupied by the spring.
In an advantageous embodiment of the invention, the compression spring is configured as a helix spring, one end of which pushes against a support surface located on the material portion forming the second part and facing against the direction of the force introduction. The other end of the helix spring pushes against the surface (the latter facing in the same direction as the force introduction) of a support shoulder of a bolt passing axially and with lateral play through the material portion of the second part and through the helix spring along the direction of the force introduction. The bolt is anchored in the first part and axially moveable relative to the second part against the pre-tensioning force of the compressive spring. According to this configuration, the bolt which is firmly connected to the first part and moveable relative to the material portion that forms the second part extends with lateral play inside the helix spring in the direction parallel to the force introduction. The purpose of the lateral play is to assure that the bolt does not obstruct the mobility of the first part in relation to the second part. The material portion that forms the second part contains the support surface facing against the direction of the force introduction that serves to hold one end of the helix spring and thereby pushes against that end of the compression spring in opposition to the applied force. Starting out from this support surface, the compression spring surrounds the shaft of the bolt with lateral play and extends to the support shoulder of the bolt that faces in the direction of the force introduction, i.e., opposes the support surface of the material portion forming the second part. When a force in excess of the nominal load capacity (as set by the degree of pre-tension in the compression spring) is introduced into the first part, the compression spring is being further compressed, causing the support shoulder of the bolt to move towards the support surface of the second part, in other words causing a displacement of the first part relative to the second part in the direction of the force introduction.
In the same context, a further development of the invention provides for a cavity inside the second part to enclose the circumference of the helix spring. This cavity may be produced in a simple manner in the form of a bore hole with the same axial direction as the force introduction that partially traverses the material portion forming the second part as well as the guide member adjoining the second part on the side from which the force is being introduced, in which case the bottom of the bore hole forming the cavity at the same time serves as the retaining surface for the helix screw.
In a further developed practical version, the cavity in the material portion forming the second part has a stop that limits the axial displacement of the bolt. This stop could, e.g., be provided by the bottom end of the blind hole that forms the cavity, with a gap between the bottom of the blind hole and the end portion of the bolt that points in the direction of the force introduction, thereby providing a displacement-stopping rest position for the end of the bolt. This stop limits the range of travel of the bolt in the absence of other restraints such as will be available after assembly of the force-measuring device, e.g., through the displacement-stopping contact of a balance pan support connected to the load receiver against a rest stop on the enclosure.
In a further practical design alternative, the end portion of the bolt that points in the direction of the force introduction protrudes out of the surface of the material block. After assembly, this protruding end portion can function together with an enclosure-based rest stop to provide a travel restraint.
In the same context, it is practical for the bolt to have a connector portion for receiving the force to be measured at the end of the bolt facing against the direction of the force introduction. In this case, the bolt guiding the helix spring at the same time serves the purpose of receiving the force that is to be introduced into the first part. For an application in a balance, the connector portion of the bolt preferably has the shape of a conical support peg on which the balance pan is seated.
In an advantageous spatial configuration of the preceding embodiments, the bolt is arranged in the portion of the load receiver that extends between the two guide members. The advantage lies in minimizing the required amount of space, given that the spatial dimension dictated by the guide members is at the same time used to accommodate the bolt.
Also preferred is a design where the material-free space is at least in part formed by only a thin linear cut traversing the material block. The width of the thin linear cut can be reduced to the minimum amount that is required to still allow a sufficient displacement of the first part relative to the second part in the case of an overload condition. Thin linear cuts of this kind can be produced primarily through the process of spark erosion with practically no limitations as to their shape. The thereby achievable cutting widths are as small as, e.g., a few tenths of a millimeter. Suitable raw materials for the material block are, e.g., aluminum alloys, but numerous other raw materials may also be considered including, e.g., steel alloys or composite materials.
Advantageous embodiments of the invention are distinguished by the fact that the two guide members on the sides that face each other are contoured by sections of the thin linear cut. A thin flexible portion is delimited at each end of each guide member between the linear cut section and the respective opposite, outward-facing side of the guide member. Thereby, a parallelogram mechanism is formed that guides the first part of the load receiver, the corners of the parallelogram being defined by the thin flexible portions.
In a further practical development of the aforementioned embodiments, the thin linear cut has a section starting at the end of the first shoulder nearer the first part and ending at the terminal segment of a section that delimits the first guide member near the end closer to the first part. The thin linear cut further has a section starting at the end of the first shoulder nearer the second part and ending at the terminal segment of a section which delimits the second guide member near the end closer to the first part. Thereby the sections of the thin linear cut extending from the shoulders towards the guide members, together with the shoulders that extend in the transverse direction relative to the force introduction, define the shape of the contours of the first part and the second part facing each other in a complementary manner such that in particular the material portion forming the second part extends between the two guide members towards the material portion that forms the first part. In this arrangement, the shoulders extending in the transverse direction relative to the force introduction are located in the area of the force transducer that extends roughly between the thin flexible portions of the guide members that are nearer to the first part. The elastic member that urges the two shoulders into mutual contact is arranged, e.g., next to the shoulders on the side of the load receiver that faces away from the first part.
In a special configuration of this arrangement, the sections of the thin linear cut that delimit the guide members are at least in part wider than the section that connects the sections delimiting the guide members. The widened portions are arranged such that they will allow an increased amount of displacement travel of the first part relative to the second part.
It is also within the scope of the invention that the material portion forming the second part is guided in parallel motion relative to a stationary part of the force-measuring device by two parallelogram guides that extend lengthwise in the direction transverse to the force introduction and are rigid in their longitudinal but elastically flexible in their transverse direction, each of the guide members being connected at one end to the material portion forming the second part and at the opposite end to the stationary part of the force-measuring device, and that the material portion forming the second part is coupled to a mechanism for transmitting the force to be measured to the transducer. As long as the nominal capacity load is not exceeded, the first part and the second part of the load receiver remain rigidly coupled to each other, so that the load receiver is guided in a translatory displacement in the direction of the force by the parallelogram guides. This translatory displacement, which is caused by the force or load to be measured and which can be an immeasurably small or virtual displacement, is transferred to the transmitting mechanism for the force to be measured and allows the latter to be transmitted to the measuring transducer. As the first part and the second part are uncoupled from each other when the nominal capacity load is exceeded, the force-transmitting mechanism as well as the measuring transducer are protected against excessive inertial forces.
A particularly advantageous embodiment that has been further developed in this regard is distinguished by the fact that the parallelogram guides and the stationary part are formed as material portions of a material block that are monolithically connected to the second part and are separated from each other by a material-free space that traverses the material block. Thereby the advantages of a monolithic construction are being realized not only with respect to the overload protector but also with respect to the guiding restraint of the load receiver that is required by the measurement process.
With preference, this embodiment is configured in such a manner that the parallelogram guides at their longitudinal terminations have flexible portions that are delimited by material-free spaces. Particularly practical is an arrangement where at least one of the material-free spaces delimiting a flexible portion is formed by a thin linear cut that opens to an outside border of the material block and from that open end forms a curve that tangentially approaches the longitudinal direction of the parallelogram guides.
In the further expanded configuration of this embodiment, the force-transmitting mechanism has at least one lever, one arm of which is coupled to the material portion forming the second part by a coupling member extending in the direction of the force introduction that is rigid in its lengthwise direction and elastically flexible in its transverse direction. The force to be measured, after it has been introduced to the load receiver, is reduced or increased by the at least one lever to a magnitude that is appropriate for the measuring transducer.
In this case again the advantages of monolithic construction are realized in that the coupling member and the lever are formed as integrally connected material portions bounded by material-free spaces in a material domain of the stationary part that reaches out into the space between the two parallelogram guides. Again with respect to minimizing the required spatial dimensions and increasing the strength of the parts of the force-measuring device that are subjected to the force to be measured, it is advantageous if the material-free spaces delimiting the coupling member and the lever are at least in part formed only by thin linear cuts dissecting the material block.
Finally, it is particular to the configuration of all of the inventive embodiments that the material block has essentially the shape of a rectangular block, of which the largest pair of surfaces extends in parallel to the lengthwise direction of the parallelogram guides. In this, the material-free spaces traverse the material block from one to the other of the two largest sides of the block in a direction parallel to one of the smaller boundary surfaces of the block, whereby a compact and overall block-shaped design configuration with no protruding parts is achieved.
Further distinctive features, details and advantages of the invention will become evident from the following description and from the drawing that is also being referred to for the disclosure of all details essential to the invention that are not expressly mentioned in the text.